Meteorological winter is a three month period that runs from Dec 1st to the end of February. It is the coldest three month period of the year in the northern hemisphere. Astronomical winter is what we all refer to when we talk about the winter season and this is based on when the sun reaches the most southern point on the globe, the Tropic of Capricorn. If you are located right on the Tropic of Capricorn at 12:00 noon on the first day of astronomical winter, the sun will be directly overhead. Also, on the first day of astronomical winter, the sun is at its lowest point in our sky at 12:00 noon and, of course, it is the shortest day of the year. Obviously, it is the first day of the summer season in the southern hemisphere.